This invention relates to a vapor-compression air conditioning system embodying a liquid desiccant for simultaneously cooling and dehumidifying conditioned air.
Liquid desiccant system can provide cooling where no active cooling is available by drying the air to a level below that required for comfort conditions, exchanging heat with the ambient environment, and then injecting moisture into the system. However, desiccant systems requires low ambient wet bulb temperatures to produce the requisite cooling. In contrast, vapor-compression systems must actively cool the air below the dew point of the air entering the evaporator in order to dehumidify the air by condensation. The vapor-compression system thereby requires that evaporator temperature be driven to a level much lower than required to achieve sensible cooling.
Hybrid vapor-compression, liquid desiccant systems combine the benefit of both desiccant systems with vapor-compression systems. Hybrid systems combine active, sensible cooling inherent in vapor-compression systems with passive, latent cooling inherent in desiccant dehumidification systems. The hybrid system need not be supercooled in order to remove moisture from the system. Consequently, energy is not wasted over-conditioning the air because moisture is sorbed rather than being condensed from the air being conditioned.
Hybrid vapor-compression, liquid desiccant systems operate by sensibly cooling the air and sorbing the moisture from the air. Sensible cooling occurs by circulating compressed and expanded refrigerant between an evaporator and condenser found in a standard vapor-compression system. Dehumidification occurs by contacting air with a desiccant on mass exchange surfaces. The mass exchange surfaces are sprayed with a liquid desiccant as outdoor air, air returning from the conditioned space, or a mixture of both, are drawn or blown through the mass exchange surfaces. The mass exchange surfaces described in prior art are separated from the heat exchange surfaces of the vapor-compression system. Conventional mass exchange surfaces often require a separate heat exchange surface for pre-cooling or pre-heating desiccants prior to being sprayed into the mass exchanger. The problems associated with separate heat and mass transfer surfaces are increased costs required to purchase separate heat and mass exchangers and reduced thermal and mass transfer efficiencies.
In the dehumidification process, moisture is sorbed from conditioned air by spraying and cooling a desiccant contacting the air in a sorbing mass exchanger or sorber. Water is sorbed in direct contact with sprayed droplets of desiccant entrained with air or on falling films of desiccant covering part or all of the mass exchange surface of the sorber. Conventional spraying techniques are inefficient methods for dehumidifying air because spraying creates an adiabatic sorbing process which increases the temperature of the sorbent, thereby reducing mass transfer. Thus, conventional spraying means require cooler exchange surfaces and produce a less efficient system because cooling is required to remove the heat of condensation, the heat of solution, and the sensible heat transferred from the air being conditioned. Conventional hybrid system waste energy by also having to transfer heat by heat exchange means external to the heat exchanger surfaces of the vapor-compressor system, or by circulating the desiccant through the heat exchange surfaces of the vapor-compression system.
During heat exchange, the desiccant solution is diluted with water and falls by gravity to a sump or reservoir placed within or below the sorber. To maintain a dehumidification process, the diluted desiccant must be desorbed, i.e., regenerated. Regeneration accomplished by spraying and heating the diluted desiccant in contact with air expelled from a desorbing mass exchanger or desorber. Consequently, a portion of the diluted desiccant in the sump of the sorber is pumped to the desorber for concentration. Water is desorbed from the sprayed droplets of desiccant entrained with air or by falling films of desiccant covering part or all of the mass exchanger surfaces of the desorber. Heating is required to provide the heat of vaporization necessary to evaporate water from the desiccant solution and to heat the air contacting desiccant solution. The heat is provided by a primary energy source such as natural gas or electricity, or a renewable energy source such as solar, waste heat or any combination of these sources. When waste heat from the vapor-compression system is reclaimed, the heat is transferred by heat exchanger means external to the heat exchange surfaces of the vapor-compression system, or by circulating the desiccant throughout the heat exchange surfaces of the vapor-compression system. The desiccant solution is concentrated during this process and falls by gravity to a sump within or below the desorber. Continuous dehumidification is facilitated by pumping the same mass flow rate of desiccant from the sump of the desorber to the sorber as was sent from the sump of the sorber to the desorber.
Hybrid vapor-compression liquid desiccant systems that reclaim waste heat for partial or full generation of the desiccant are more efficient systems than those that use primary energy or alternative energy for regeneration. Furthermore, hybrid vapor-compression liquid desiccant systems that are configured for low-temperature regeneration are more efficient than those systems that regenerate at higher temperatures. Conventional hybrid systems incorporating spray delivery means require higher regeneration temperatures, thereby reducing thermal efficiency of the system. Moreover, conventional hybrid systems which do not combine heat and mass exchange surfaces on a single surface are less efficient and require more operation energy.